Real HP loss numbers

[Shannen Durphey]

Ok, I'm working to understand this.  The chassis dyno has no brake, so
they measure the change in acceleration of the drums to compute
power.  This means that anything that changes the rate of acceleration
of the wheels changes the rate of acceleration of the drums, and the
calculated horsepower.  This also means that without acceleration, the
chassis dyno cannot measure horsepower, hence no steady rpm
measurements can be taken and no tuning done at any given rpm, yes?  

This would also mean that claims like "lighter pulley adds 5 hp" are
mrginally correct based on chassis dyno measurements, but technically
should say something like "lighter pulley increases acceleration
equivalent to adding 5 more hp", since at any given rpm the hp level
is ~constant, regardless of the mass of the rotating components.
Yes?  
I only ask because I can see no other way that claims for replacing
rotating components with lighter ones can add power.  But I sure can
see that they'll change the rate of acceleration.

Shannen 
Daniel Ciobota wrote:
> 
>  Let me jump in on this discussion with an observation.  Rear wheel hp numbers
> are measured by observing the acceleration of two 1000lb drums by the driving
> wheels of the car.  That's how hp is calculated, rotational speed vs. time,
> known weight of 2000lbs and known drum diameter.  From those numbers, and final
> gear ratios (rpm/drum rotation), the chassis dyno calculates instantaneous
> torque required to accelerate the drum at that rate.
>  The key measurement here is rotational mass.  That parameter affects how fast
> those drums will accelerate, and since it's a known quantity for the chassis
> dyno, accurate hp numbers at the driving wheels can be calculated.
>  To backtrack flywheel hp from driving wheel hp, a couple of parameters are most
> important: frictional loss, as discussed here already, and _rotational mass_,
> more accurately, momentum.  Take for example an aluminum flywheel and the
> equivalent steel version.  In my stang, the aluminum piece weighed 10lbs, while
> the steel piece was 23lbs.  In neutral, the aluminum flywheel accelerated
> significantly faster than the steel piece; no surprises there.  The same concept
> applies to the rotating pieces inside transmissions, driveshafts and rear ends;
> the total momentum affects how fast the drive wheels can accelerate, thus
> affecting rear wheel hp numbers.
>  From observation of different dynoed cars, I think that gm transmissions have
> slightly less momentum and frictional loss than their ford counterparts.  And,
> of course, because of the fluid and rather massive torque converters, automatics
> are significantly more sluggish and offer more loss.
>  So, when someone is talking about a 75hp loss, not all that loss is friction,
> otherwise our drivetrains would glow in the dark!  However, much of the loss due
> to momentum is taken up by the engine (more strain), which shows up as heat in
> the coolant and oil.
> 
>  If my physics are wrong, please correct me.
> 
>  Just my $0.02.
> 
> Daniel
> 
> Roger Heflin wrote:
> 
> > On Fri, 12 Mar 1999, Clive Apps   Techno-Logicals   416 510 0020 wrote:
> >
> > > >
> > > > I have wondered about this myself.  I have heard anywhere from 15%
> > > > to 25% in drivetrain losses.  What I can't figure out is where the
> > > > energy is going.  If you have a 300HP engine with 25% drivetrain
> > > > loss, then you are losing 75HP somewhere.  Since it doesn't just
> > > > disappear, something has to be soaking up 75HP of energy.  My
> > > > guess is that the loss would be in the form of heat which would
> > > > mean of lot of drivetrain parts would have to be getting really hot
> > > > (assuming the 300HP load on the engine).  Since I have only seen
> > > > my transmission get warm, it doesn't really make much sense unless
> > > > I just haven't had the load on for a long enough period.
> > >
> > >
> > > just estimate how much torque it takes to compress a valve spring
> > > multiply x 1/2 revs x # springs X length of valve travel x
> > > x losses in the vlave gear and add in the amount that it takes to
> > > move the valve train components around
> > > 15% sounds reasonable
> > > on 300 HP that would meav that another 52 HP was being eaten by the engine
> > > to move the valves
> > >
> > > Clive
> > >
> >
> > For the 15% number everyone has generally ment the amount of hp lost
> > after the flywheel.  The wheel dynos hp nubmer vs.  the engine only
> > dyno numbers.  So that claim is rear wheel hp * 1.15 (manual tranny)
> > is roughly equal to the engine hp at the flywheel.   The valves are
> > the same and already accounted for in both cases.
> >
> >                         Roger